This invention relates to a system for the analysis and measurement of selected gases and, more particularly, for measurement of combustible gases such as hydrocarbon gases contained in a gaseous sample to be analysed.
There is a demand for information indicating the hydrocarbon content of gaseous mixtures. For example, the return flow drilling mud material discharged from an oil or gas exploration well can contain entrained hydrocarbon gases. Detection and measurement of the hydrocarbon gas content of the well return material can be used to give an indication of when a certain zone is being penetrated in the well drilling process. Such data can provide information to the geology personnel on the drilling project to enable them to form an assessment or provide an indication as to whether the well drilling has hit a producing zone. In oil and gas exploration, the primary hydrocarbon gas of interest is generally methane, although, under certain drilling conditions, there is also interest in information relating to some of the other hydrocarbon gases that may be present.
The current state of the art uses a variety of apparatus and methods to quantify and qualify the hydrocarbon content of a gas sample, that is, to perform analysis of the sample. The simplest types of apparatus to perform analysis of a gas sample, are systems that use a xe2x80x9cthermal conductivity detectorxe2x80x9d (TCD). Thermal conductivity detectors are suitable when the gas to be analyzed by the detector contains a known gas in a known carrier gas. This is often referred to as binary analysis of gas. Every gas has a unique thermal conductivity as one of its properties. Thermal conductivity detection works best when the carrier gas and the sample gas have very different thermal conductivities. Typically, the TCD detector has a Wheatstone bridge arrangement where the detector element manifests a decrease in resistance with increasing thermal conductivity of the sample gas. By way of example, U.S. Pat. No. 3,683,671 to Van Swcray entitled Measuring System Including Thermal Conductivity Detector Means provides an electrical circuit bridge excited at one power node, by a clamped square wave arm at another power node by a feedback circuit. The output of the circuit bridge is fed to a demodulator to generate an output signal representative of the sample being sensed.
A Wheatstone bridge and visual indicator in the form of light emitting diodes in a gas analyser arrangement is disclosed in U.S. Pat. No. 4,028,057 to Nelson. These detectors are used in gas chromatography where a carrier gas that has a very high thermal conductivity, such as helium, is used. When a sample that has a much lower thermal conductivity than helium is introduced into the carrier gas the output of the detector will show a change relative to the amount of sample contained within the carrier. A thermal conductivity detector can be confused, that is produce erroneous output, if more than one type of sample gas is introduced into the carrier gas. That is if the thermal conductivity detector is used to analyse a gas mixture of multiple sample gases. For example, if one of the sample gases has a higher thermal conductivity than the carrier gas and the second sample gas has a lower thermal conductivity than the carrier gas, then the detector output may not even change for varying constituent gas compositions or mixtures.
Thus, a thermal conductivity detector is not well suited to analysis of hydrocarbon gases entrained in well returns for a number of reasons. First, it is not feasible to transport large tanks full of helium to the well site. Consequently, the carrier gas that is generally used is air. Air has a thermal conductivity of 1.00 and methane a thermal conductivity of 1.3. This means there is not a very good signal to noise ratio between the air carrier and the gas of interest, which makes a thermal conductivity detector based instrument prone to drifting. Notwithstanding their drawbacks, such thermal conductivity detectors are in use in analyzers used in the oil well drilling industry. However, because of the inherent limitations of using TCD detectors in these environments, it is not uncommon to need to zero the baseline of a TCD based system on an hourly basis. Automated baseline adjustment apparatus have been proposed to compensate for temperature changes in such systems. For example, the arrangement proposed by Hagen in U.S. Pat. No. 4,817,414.
Also, thermal conductivity detectors are, by their nature, sensitive to ambient temperature. Even a 1 degree shift in ambient temperature will cause a noticeable shift in the baseline of a thermal conductivity detector operating in this low signal to noise ratio configuration.
Another, somewhat more sophisticated detection apparatus employs a catalytic combustion detector (CCD) to detect the presence of hydrocarbons. For example, U.S. Pat. No. 3,607,084 to Mackey for Combustible Gas Measurement describes passing a stream of gas containing the combustible gas analytes over a conductive metal wire coated with a think catalytic coating which is at a temperature at which oxidation of the gases is initiated. Numerous other arrangements of CCD apparatus are known for example, U.S. Pat. No. 4,045,177 to McNally, U.S. Pat. No. 4,072,467 to Jones, U.S. Pat. No. 4,111,658 to Firth et al, U.S. Pat. No. 4,123,225 to Jones et al, and U.S. Pat. No. 4,313,907 to McNally are examples of such CCD detectors. CCD""s are sensitive to anything that is combustible and in an oil and gas well drilling environment, hydrocarbon gases are the combustible gases that would be encountered. This means a CCD can be used as to provide a measurement of the total hydrocarbon content of a gas without regard to the particular type of hydrocarbon gas. While a CCD will respond to combustible compounds other than hydrocarbons, it is the gaseous hydrocarbon compounds that will be of interest in the sample gases recovered from the drilling mud in a well drilling environment. A major problem with CCD""s is their limited range. If a CCD is subjected to explosive combustible gas concentrations, that is concentrations between the upper and lower explosive limits of that compound, they are destroyed as the gas actually combusts and coats the detector surface with carbon, rendering it ineffective after that point. For methane the lower explosive limit is 5% in air. An air mixture containing methane gas concentrations greater than the 5% lower explosive limit will result in a mixture that becomes explosive.
To obtain the benefit of a stable baseline and wider range of methane concentrations in a sample, two detector systems have been produced. Current state of the art two-detector apparatus uses a CCD sensor to around 4% concentration in the mixture. Above that point, the sensor apparatus control turns off the CCD sensor and passes the sensing over to a thermal conductivity sensor. A thermal conductivity sensor, of course, has all of the problems as described above. However, a major advantage of a two-detector analyser is a more stable baseline.
A combined CCD and thermal conductivity analyzer has some major drawbacks if a gas other than methane is present in the sample to be analyzed. For instance, if C2 is the gas being presented to the CCD, the CCD will detect its presence very nicely. However, when the analyzer switches over to the thermal conductivity detector, the C2 gas may not be detected at all. The system will respond by switching back to the CCD which ultimately causes the system to keep switching back and forth between the two sensors and can result in the destruction of the CCD due to exposure to explosive levels of C2 gas in the sample. An example of a two-detector system is shown, for example, in U.S. Pat. No. 4,804,632 to Schuck et al which switches from one sensor to another based on set sample temperatures and holding the sensing devices to a preset temperature.
Another gas detection system using a CCD detector, operates by diluting the sample with air when it exceeds 4% as shown, for example, in U.S. Pat. No. 3,771,960 to Kim et al. Adding diluting air to the sample allows such a gas detection system to use a CCD sensor throughout the entire range. Generally, such gas detection system apparatus provides preset ranges, for example 0% to 3% which is the undiluted range and a second dilution range, for example 0% to 100%. In one prior art arrangement, the dilution is accomplished by using a manifold with orifices drilled into it that give approximate volumes of gas for the dilution blending. An on/off valve is used to control the diluting of the sample with air. This system requires precise adjustment of needle valves in the factory before being shipped. A problem with this dilution approach is that gas concentrations vary considerably with pressure and temperature and thus are very hard to control precisely enough to give an accurate reading when there is a switch over from one range to the other. In addition to the pressure temperature aspects of the dilution blending problem, a further problem inherent in this method is that the dilution is very hard to effect without either reducing the sample drawn from the extraction device or increasing the amount of sample passed through the detector.
In conventional combustible gas analysers, a constant flow rate through the detector is maintained by reducing the amount of sample drawn from the sample source or extractor. On the other hand, where a constant flow rate from the sample source or extractor is maintained, an increase in the flow rate through the detector is caused by the air added to or blended with the sample to produce the diluted mixture flowing through the detector. Neither of these situations is optimal. Drawing less sample gas from the gas trap or sample extractor can cause the concentrations to rise as the gas trap is extracting gas from the drilling mud at a certain rate. If the rate of sample extraction is suddenly reduced, then there will be a build up of sample gas inside the extractor. On the other hand, if the extraction rate is kept constant, the addition of diluting gas will cause the volume of the diluted sample gas mixture produced to increase with a corresponding increase in the sample flow rate through the detector. Changes in sample flow rates through a CCD detector will consequently change the response of the detector, as the detector response is dependent on sample flow rates to the detector. To give accurate results, CCD detectors require a precise flow rate. In operation, a CCD detector actually destroys sample that it comes in contact with, so, at low flow rates, the readings will drop off as there is more and more dead sample in contact with the detector.
To overcome these shortcomings, in one of its aspects, the invention provides a sample gas dilution system to control the supply of a sample gas to a detector supply port for supply to a sample detector system. The gas sample dilution system is arranged with three gas flow controls. A sample gas flow control is provided to control input sample gas flow to a detector supply port. A diluting gas flow control is provided to control supply of a diluting gas to the detector supply port and therefore control blending of the sample gas with the diluting gas. An exhaust flow control is provided to control an exhaust flow of excess sample gas not required by a sample detector system coupled to the detector supply port. A controller, such as a computer, provides the settings of the flow controls. In the preferred manner of operation, the controller operates the flow controls to keep the input sample gas flow rate into the sample dilution system constant and the gas flow to the detector supply port constant. That is, the controller operates the sample gas flow control, the diluting gas flow control, which controls blending of the sample gas with a diluting gas supply, and the exhaust flow control which controls an exhaust flow of the sample gas to maintain a constant input sample gas flow rate from the gas sample source and a constant output flow rate to the detector system. Excess sample gas not required for supply to the sensor block of the sample detector system is exhausted from the apparatus.
In the preferred embodiment, each gas flow control has a proportional control valve responsive to a control signal to control the flow of gas therethrough. Preferably closed-loop controlled mass flow controls are utilized to facilitate precise control of gas quantities and flow rates. In a closed-loop controlled gas flow control, the gas flow control includes a flow sensor to produce signalling representative of the gas flow rates therethrough. The flow sensor provides a feedback signal that is used in the control of the proportional control valve to facilitate closed-loop control of the proportional control valve based on feedback from the flow sensor.
In another aspect of a preferred embodiment of the invention, the sensor block or detector system operates in conjunction with the sample dilution system to allow for several ranges to be implemented yet keep the signal to noise ratio from the detector devices at optimum levels. One preferred embodiment discloses ranges of 0% to 4%, 0% to 8%, 0% to 16%, 0% to 32%, 0% to 64% and 0% to 100%. An algorithm for automatic range selection permits optimal sensor block utilisation with minimal user intervention while providing an output representative of combustible gas concentrations in the sample gas without the need to configure or reconfigure the instrument manually.
In one of its aspects, the invention provides an apparatus for mixing gases comprising a manifold forming a diluting gas port, a sample inlet port and a detector supply port all in common communication with each other. A diluting gas flow control means is provided which is operable to control a flow of diluting gas through the diluting gas port in response to a first control signal. A sample gas flow control means is operable to control a flow of sample gas to the detector supply port in response to a second control signal. A detector means in communication with the detector supply port is operable to produce output signalling representative of the content of a selected gas of a gas mixture passing therethrough. A control means is provided to produce the first and second control signals for respective diluting gas and sample gas flow control means whereby any gases supplied to the manifold are mixed therein and expelled through the detector supply port in proportions set by the control means.
In another of its aspects, the invention provides an apparatus for mixing gases comprising a manifold forming a sample gas inlet port, an exhaust port, a diluting gas inlet port and a detector supply port all in common communication with each other. A diluting gas flow control means is operable to control a flow of gas through the diluting gas inlet port in response to a control signal. A sample gas flow control means is provided to control a flow of sample gas to said detector supply port in response to a control signal. An exhaust gas flow control means is provided to control a flow of gas through the exhaust port in response to a control signal. A control means includes means to receive a detector signal output, the control means produces a respective control signal for the diluting gas flow control, sample gas flow control and exhaust gas flow control means is included whereby a constant rate of gas flow through said detector supply port is obtained. The sample gas supplied to the sample gas inlet port and the diluting gas supplied to the diluting gas inlet port are mixed and expelled through the detector supply port in proportions set by the control means responsive to a received detector signal output.
And in yet another of its aspects, the invention provides a method of measuring a gas mixture comprising: receiving a sample gas from a source at a predetermined sample gas flow rate, supplying a gas mixture to a detector at a predetermined detector supply gas flow rate and receiving a detector signalling produced by a detector monitoring the supplied gas mixture. Periodically the received detector signalling is compared to a predetermined range. A supply of diluting gas is mixed with a selected portion of the sample gas flow to supply the gas mixture at the predetermined detector supply gas flow rate and yet maintain the received detector signalling within the predetermined range.
Preferred embodiments of the invention will now be described with reference to the attached drawings. For convenience, like reference numerals have been used to depict like elements of the invention throughout the various drawings.